The application generally relates to variable speed drives. The application relates more specifically an electronics power assembly in a variable speed drive.
A Variable Speed Drive (VSD) is a system that can control the speed of an alternating current (AC) electric motor by controlling the frequency and voltage of the electrical power supplied to the motor. VSDs may be used in various applications, for example, ventilations systems for large buildings, pumps and machine tool drives.
A VSD incorporates several stages to provide speed control to a motor. A VSD may include a rectifier or converter stage, a DC link stage, and an inverter stage. The rectifier or converter stage, also known as the converter, converts the fixed line frequency, fixed line voltage AC power from an AC power source into DC power. The DC link stage, also known as the DC link, filters the DC power from the converter and typically contains a large amount of electrical capacitance. Finally, the inverter stage, also known as the inverter, is connected in parallel with the DC link and converts the DC power from the DC link into a variable frequency, variable voltage AC power.
When electric power is applied to the VSD, the voltage across the DC link capacitors, referred to as the DC link voltage, rises from zero to a rated value. If the rise of the DC link voltage were left to occur uncontrolled, the rise in voltage level would occur very quickly by drawing very large electric currents from the AC power source through the rectifier and into the DC link capacitors. The large current drawn by the DC link capacitors, referred to as an inrush current, can be damaging to the components of the VSD. Thus, to avoid damage to the VSD components from inrush current, the rise of the DC link voltage from 0 V to the rated voltage should be controlled. The control of the DC link voltage is referred to as a DC link precharge operation, or precharge, of the circuit.
In some VSD applications that employ controlled charging, or precharging, of the capacitors to limit the inrush current, the converter is arranged to provide the precharge current to the DC link. The conduction of the semiconductor devices, such as insulated gate bipolar transistors (IGBTs) or other types of power switches or transistors used for rectifying the AC line voltage, is controlled so as to let only small pulses of inrush current flow during precharge operation of the VSD. The semiconductor devices used for controlling the inrush current during precharging are usually turned on all the time afterwards. These semiconductor devices used only during precharging can have a maximum rated current rating that is less than the maximum rated current of the main semiconductor devices.
The converter may be subject to harmful or destructive currents in the occurrence of a fault or short circuit condition on the input of the converter or output of the inverter. The harmful or destructive currents from a fault or short circuit can damage components of, or the entire VSD. Therefore, it is desirable to provide protection to the VSD during short circuit conditions to prevent damaging the semiconductor devices of the converter and other components of the electrical distribution system and the VSD.
The use of auxiliary semiconductor devices with lower maximum current rating along with higher current rated main semiconductor devices in the converter poses a problem to safely turn off during the short circuit or fault event. What is needed is a scheme to improve the reliability of such variable speed drive during short circuit or fault events.